The present invention relates to a gas flow meter for automotive control and more particularly to a noise reduction circuit, to an adjustment circuit, to a reduction in the number of adjustment terminals and output terminals, and to an output circuit.
A gas flow meter for detecting an air flow in internal combustion engines has been in use. An example of the gas flow meter is a constant temperature control hot wire type gas flow meter described in the Journal of Fluid Mechanics, vol. 47 (1971), pp577-599. FIG. 25 shows an outline configuration of a gas flow detection circuit DECT1 applying the constant temperature control heat wire type gas flow meter. This gas flow detection circuit mainly comprises an operational amplifier OP1, a power transistor Tr1, a heating resistor (also called a hot wire) Rh, a gas temperature measuring resistor (also called a cold wire) Rc and resistors R1, R2 and keeps the temperature of the heating resistor Rh constant at all times, i.e., keeps its resistance constant by maintaining a bridge balance using the operational amplifier OP1. As the gas flow increases, heat taken from the heating resistor Rh increases resulting in an increased heating current. Because this heating current is proportional to a voltage between terminals of the resistor R1, the measurement of this voltage can determine the gas flow. The voltage output produced by the current detection resistor R1 is processed by an adjust circuit having a predetermined input/output characteristic so that the voltage output provides a predetermined signal characteristic required of the gas flow meter.
There is another gas flow detection circuit DECT2, as shown in FIG. 26, in which heat sensing resistors Ru, Rd for measuring gas flow temperatures are arranged upstream and downstream of the heating resistor Rh of the constant temperature control hot wire type gas flow meter so that they are influenced by heat from the heating resistor Rh. The resistor Ru on the upstream side is cooled by the gas flow to lower its resistance and the resistor Rd on the downstream side receives a gas flow heated by the heating resistor Rh to raise its temperature and therefore its resistance. This changes the potential at a connecting point between Ru and Rd and thus measuring this voltage can determine the gas flow.
Still another gas flow detection circuit DECT3 as shown in FIG. 27 is available, in which a total of four heat sensing resistors for measuring gas flow temperatures are arranged two upstream and two downstream of the heating resistor Rh of the constant temperature control hot wire type gas flow meter so that they are influenced by heat from the heating resistor Rh, and in which one pair of resistors Ru1, Rd1 are serially connected in an upstream-downstream order and another pair of resistors Rd2, Ru2 are serially connected in a downstream-upstream order to form a bridge and measure a potential difference between two connecting points. The resistors Ru1, Ru2 on the upstream side are cooled by the gas flow to lower their resistances and the resistors Rd1, Rd2 on the downstream side receive a gas flow heated by the heating resistor Rh, raising their temperatures and therefore their resistances. This changes the potential difference in the bridge and thus measuring this voltage difference can determine the gas flow.
The electronic circuits that adjust the output characteristic of a gas flow meter mounted on motor vehicles are subject to various surges and overvoltages, as specified in the International Standard Organization (ISO) 7637-1, 7637-3 standard and Japan automotive standard (JASO) D001-94. These standards are intended to prevent undesired operations or failures of electronic circuits due to surge voltages caused by ignition of engine, overvoltages caused by batteries stacked in two tiers at time of starting engine in cold environment, and high frequency noise caused by other electronic devices. On the other hand, the electronic circuits are constructed in the form of IC circuits for reducing the manufacturing cost and, in recent years, to meet the emission control requirements the gas flow meter is increasingly required to raise its precision in line with the sophistication of engine control functions. Further, because the service temperature range is as wide as −40° C. to 130° C., measures should be taken to prevent a possible change in output due to temperature variations.
For surges and overvoltages, a variety of overvoltage Protection circuits have been in use. One such example is a protection circuit using a Zener diode ZD and a current limiting resistor R as shown in FIG. 28.
The circuit of FIG. 28 is one type of a commonly used constant voltage circuit in which a voltage applied to a connection terminal VBB for the battery causes a current to flow through the current limiting resistor R to the Zener diode ZE. When an overvoltage is applied, the voltage of the power supply terminal Vcc to various circuits is clamped by a Zener voltage of the Zener diode ZD to put an overvoltage protection into action.
Further, JP-A-9-307361 proposes as a conventional technology an overvoltage protection circuit that uses an overvoltage detection circuit made up of a resistor and a Zener diode and a switching circuit made up of bipolar transistors.
The overvoltage protection circuit described in this official gazette is intended for protecting microwave FETs (field-effect transistors). When an overvoltage higher than a voltage sum of the Zener voltage of the Zener diode and the base-emitter voltage of the switching transistor is applied to the power supply terminal, the switching circuit is operated to cut off the load from the power supply line and thereby prevent the overvoltage from being impressed on the load.
The voltage outputs of the flow detection circuits DECT1-3 in FIG. 25 to FIG. 27 need adjustments in zero point and span (output range) to produce the required sensor output characteristics. This adjust circuit is mainly an analog circuit at present but a higher precision adjustment is considered possible by using a digital circuit.
Table 1 shows comparison between an analog circuit and a digital circuit (“CMOS Analog Circuit Design Technique” published by Triceps (1998), compiled under the supervision of Iwata).
TABLE 1Analog circuitDigital circuitNo. ofFew (about 20 pcs inMany (2000 pcs intransistorsmultiplier)8-bit multiplier)Chip areaSmall (few devices)Large (manydevices)PowerLow powerLarge (many gatesconsumptionconsumption becauseare switched)of fewer devicesClockLow (determined byHigher (½ of cut-frequencysettling ofoff frequency ofamplifier)device)SignalHigh (about ½ ofLow ( 1/10 of clockfrequencycut-off frequency offrequency)device)PrecisionLow (deviceHigh (depending ondeviation, noise)bit number)StabilityLow (oscillation,Highcharacteristicvariation)NoiseLow (S/N)Strong (large noiseresistancemargin)Source: “CMOS Analog Circuit Design Technique” published by Triceps (1998), compiled under the supervision of Iwata 
The analog circuit has a small size and a small power consumption compared with the digital circuit. But the use of such devices as resistors causes manufacturing variations and other variations due to aged deterioration, and thus the analog circuit has less precision and stability than the digital circuit. The digital circuit, while it is superior to the analog circuit in terms of precision and stability, has a larger circuit size and a larger power consumption. The rapid advance in the integrated circuit manufacturing technology in recent years, however, has enabled micro-fabrication and therefore reduced the circuit size and power consumption. The digital circuit is now finding many applications in various industrial fields. Example applications of a digital adjust circuit to the gas flow meter are found in Japanese Patent No. 3073089 and JP-A-8-62010 and JP-A-11-118552.
FIG. 29 shows comparison between an analog adjustment and a digital adjustment in the adjust circuit of the gas flow meter.
An outline circuit configuration for analog adjustment shown in FIG. 29 comprises an operational amplifier OP2, trimming resistors Rs1, Rz1 and resistors Rs2, Rz2. This circuit trims the trimming resistors Rz1, Rs1 to adjust the voltage output from the flow detection circuit DECT and thereby adjust the zero point and span to produce an output for a desired gas flow. As the trimming resistors Rs, Rz, thin-film resistors printed on a hybrid IC or thin-film resistors on IC may be used. In trimming the resistors, a laser trimmer may be used. The laser trimmer has a disadvantage that trimming with high precision takes time and re-trimming cannot be done. Further, because only a two-point adjustment is made, it is difficult for the laser trimmer to make a complicated adjustment on the output characteristic, such as a non-linear adjustment. In the analog circuit, when the output specification for the gas flow is changed, the resistance value needs to be redesigned and, in some cases, it is necessary to redesign the hybrid IC substrate pattern, which in turn increases the man-hour of designing works.
In the case of the digital adjust circuit of FIG. 29, since the output specification can be changed by simply changing an adjust coefficient while leaving the circuit pattern intact, the number of design steps can be reduced. As an example digital adjust circuit, a method described in Japanese Patent No. 3073089 has been proposed. A rough circuit configuration for the digital adjustment is as follows. The voltage output from the flow detection circuit DECT is converted into a digital value by an analog-digital converter AD. Based on the digital value, a digital processor CALC calculates the zero point and span adjustments, which are then converted by a digital-analog converter DA into an analog signal which is an analog output for a desired gas flow. The adjust coefficient used in this calculation is stored in a storage device MEM such as PROM. Further, the digital processor CALC, because of its ability to easily perform non-linear calculations, can make non-linear adjustments as well as zero point and span adjustments during the output adjustment. With this non-linear adjustment, the adjustment accuracy is within ±2%.
Another example configuration for the digital adjustment is found in JP-A-11-118552. While its configuration is similar to that of the digital adjust circuit of FIG. 29, this circuit reduces its circuit size by using an oversampling type analog-digital converter including a delta-sigma modulator as an analog-digital converter AD.
Still another example configuration for the digital adjustment is found in JP-A-2000-338193. The adjust coefficient used by the processor in executing the adjustment calculation is written into a storage device such as PROM through a terminal of a digital input/output circuit that communicates with external circuits of the sensor. This official gazette describes that a third-degree polynomial is used for the adjustment calculation.
A further example configuration for the digital adjustment is found in JP-A-11-94620. This circuit converts a flow signal from the gas flow detection circuit into a rectangular wave signal and counts up a counter at a certain rate only while the rectangular wave is “1”. To this count value is added the adjust coefficient to produce an output.
Because the heating current flowing through the heating resistor Rh is not affected by voltage variations in the power supply (for example, battery), the voltage output of the gas flow detection circuit DECT1 has a non-ratiometric characteristic. As output specifications of the gas flow meter, there are ratiometric analog and digital output specifications in addition to the non-ratiometric analog output specification. A circuit configuration that realizes the ratiometric analog output circuit is described in JP-A-2-85724. This circuit divides an external ratiometric output reference voltage into smaller voltages by two resistors and inputs the divided voltages to an operational amplifier to realize a ratiometric output. With a sum of the two resistors set to about 10 kilo-ohm, the current to be supplied from the reference voltage is relatively small at about 0.5 mA. An example of the digital output circuit is disclosed in JP-A-8-247815. This circuit configuration comprises at least a constant temperature control circuit, a zero point/span adjust circuit and a voltage control oscillator, all integrated into one chip.
Another configuration is described in JP-A-5-203475 in which an analog output and a digital output are produced by a single circuit board. In this configuration, a single circuit board is provided with both an analog output terminal and a digital output terminal, and both analog and digital outputs are supplied to an output connector which selects and uses one of the two output signals or only one of the outputs is connected through wire to the output connector.